Environmental factors and underlying mechanisms of tree community assemblages of pine-oak mixed forests in the Qinling Mountains, China

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Understanding the mechanisms of assembly of tree communities is very important for restoring and managing pine-oak mixed forests in the Qinling Mountains, China, but the essential mechanisms remain largely unexplored. The objective of this research was to uncover the underlying mechanisms of species coexistence and to identify the key environmental factors influencing the tree community assemblages in these forests. We investigated tree species and 15 environmental factors of topography, soil properties, and stand development of pine-oak mixed forests at an elevation of 1000-2000 m a.s.l. in the Qinling Mountains. Six classical models for the distribution of species abundance were used to fit the observed distributions; a clustering analysis was conducted to divide the ecological species groups, and a redundancy analysis examined the relationship between species assemblages and various environmental factors. Zipf-Mandelbrot, neutral-theory, log-normal, and Zipf models performed well in fitting the patterns of species-abundance distribution in the pine-oak mixed forests, which was related to the complexity of the community structure of the forests. A special combination of the Zipf-Mandelbrot and neutral-theory models, however, best explained the mechanism of species coexistence for the forests and indicated that these forests were progressive successional communities able to maintain stable development during succession. In addition, multiple factors controlled the tree community assemblage of pine-oak mixed forests in the mountainous regions, although available potassium, slope aspect, average tree DBH, and slope position were significant environmental variables.